The objectives of this research are to: (1) determine if the presence or absence of specific endogenous viral (ev) loci has an effect on the development of neoplasms either alone or have a co-carcinogenic effect on other oncogenic agents; (2) determine if these loci have an effect on reproductive fitness or immune competence (3) investigate the molecular basis for restricted expression of loci that appear to represent complete retroviral genomes; and (4) develop congenic lines for these studies and other basic and applied studies of ev loci. The basic methodology will be to backcross inbred lines of chickens carrying specific ev loci to a basic inbred line lacking loci of interest, to develop semi-congenic lines. Southern blots of host DNA using viral cDNA hybridization probes will be used as the criterion for selecting breeding males. Backcross and inter-cross matings will provide progeny with and without specific ev loci for in vivo challenge with oncogenic agents, testing of immune responsiveness and reproductive fitness. Host genes regulating ev locus expression will be investigated by cloning of specific ev loci and their flanking sequences, restriction enzyme mapping and performing transfection experiments to attempt to identify host regulatory genes.